Why is Promise<void> accepted as void

[TarSzator]

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Comment

I am looking for an official explanation why TS accepts Promise as void.
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When I provide a higher-order function that accepts a callback () => void I do not want user's to inject a function () => Promise<void> because I do not handle the rejection case of the Promise inside my higher-order function which might lead to an unhandled promise rejection.

Same goes from the other direction if by accident I put a callback () => Promise<void> into an higher-order function that only accepts () => void I would like to be informed that the higher-order function will not handle the Promise rejection correctly because it does not expect a Promise to be returned by the callback but void.

But I assume there was a thought process behind the decision to accept Promise as void but I could not find in the official reason.

[MartinJohns]

This is working as intended. A callback returning void means "can be whatever, I don't care". If the user wants to return a promise, so be it. When they want to return an object, so be it.

https://github.com/microsoft/TypeScript/wiki/FAQ#why-are-functions-returning-non-void-assignable-to-function-returning-void

See also: #42709

[Jamesernator]

Same goes from the other direction if by accident I put a callback () => Promise<void> into an higher-order function that only accepts () => void I would like to be informed that the higher-order function will not handle the Promise rejection correctly because it does not expect a Promise to be returned by the callback but void.

Typescript-eslint has a rule for this if you really want to ensure you're not using this behaviour.

[TarSzator]

Thank you @MartinJohns that helped

Thanks also @Jamesernator I knew about that but good that it is written here as well for others 😄